Electrophotographic image forming apparatus (hereinafter also referred to simply as “electrophotographic apparatus”) have been often used, for example, in copying machines, printers, or facsimile units. In the electrophotographic apparatus, images are formed by way of the following electrophotographic process. At first, the surface of an electrophotographic photoreceptor provided to the apparatus (hereinafter also referred to simply as “photoreceptor”) is charged to a predetermined potential uniformly by charging means such as a charging roller. Exposure is applied to the surface of the charged photoreceptor in accordance with image information to form electrostatic latent images. The formed electrostatic latent images are developed by a developer containing a toner and the like to form toner images as visible images. The formed toner images are transferred from the surface of the photoreceptor to a recording medium such as paper and transferred toner images are fixed to form images. After the transfer operation, the toner remaining on the surface of the photoreceptor without being transferred is removed by a cleaning blade or the like. Then, the surface charges on the photoreceptor are eliminated by a light from a charge elimination lamp, by which electrostatic latent images are eliminated from the surface of the photoreceptor.
An electrophotographic photoreceptor comprises a conductive substrate made of a conductive material and a photosensitive layer disposed on the conductive substrate. As the electrophotographic photoreceptor, an inorganic photoreceptor having a photosensitive layer comprising an inorganic photoconductive material such as selenium, zinc oxide or cadmium as a main ingredient has been used generally. While the inorganic photoreceptor has basic properties as the photoreceptor to some extent, it involves a problem that the formation of the film for the light sensitive layer is difficult, plasticity is poor, and the production cost is expensive. Further, the inorganic photoconductive material generally has high toxicity and suffers from great restriction in view of production and handling.
On the other hand, since an organic photoreceptor using an organic photoconductive material has advantages that it has good film forming property for a photosensitive layer, excellent flexibility, reduced weight and good transparency and a photoreceptor showing favorable sensitivity to wavelength region over a wide range can be easily designed by an appropriate sensitizing method, it has been gradually developed as a main material for the electrophotographic photoreceptor. While the organic photoreceptor in the initial state has drawback in view of the sensitivity and the durability, such drawbacks have been improved remarkably by the development of a function separated electrophotographic photoreceptor in which a charge generation function and a charge transportation function are shared to separate materials respectively.
The function separated photoreceptor includes a layered type and a single layer type. In the layered type photoreceptor, for example, a photoconductive layer in which a charge generating layer containing a charge generating substance sharing a charge generation function and a charge transporting layer containing a charge transporting substance sharing a charge transportation function are laminated is provided as a photosensitive layer. In the single layer type function separated photoreceptor, a single photoconductive layer containing both a charge generating substance and a charge transporting substance is disposed as a photosensitive layer. The function separated photoreceptor described above also has an advantage that a selection range for the charge generating substance and the charge transporting substance constituting the light sensitive layer is wide and a photoreceptor having optional characteristics can be manufactured relatively easily.
As the charge generating substance used in the function separated photoreceptor, various substances such as phthalocyanine pigment, squarylium dye, azo pigment, perylene pigment, polynuclear quinone pigment, cyanine dye, squaric acid dye, and pyrylium salt dye have been studied and various materials of excellent light fastness and having high charge generating ability have been proposed.
On the other hand, various compounds are known for the charge transporting substances, including, for example, pyrazoline compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (e.g., refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), and Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983) and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)) and stilbene compounds (e.g., refer to Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives that have a condensed polycyclic hydrocarbon structure as the center nucleus have been developed (e.g., refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995)).
The charge transporting substances must satisfy the following requirements:    (1) they are stable to light and heat;    (2) they are stable to active substances such as ozone, nitrogen oxides (NOx) and nitric acid that may be generated in corona discharging on a photoreceptor;    (3) they have good charge transportability;    (4) they are compatible with organic solvents and binder resins;    (5) they are easy to produce and are inexpensive. Though partly satisfying some of these, however, the charge transporting substances disclosed in the above-mentioned patent publications could not satisfy all of these at high level.
Further, the charge transporting substance is required to have highly charge transportability, particularly, among the requirements described above. For example, in a case where the charge transporting layer in which a charge transporting substance is dispersed in the binder resin forms the surface layer of the photoreceptor, particularly high charge transportability is required for the charge transporting substance in order to ensure a sufficient light sensitivity.
In a case where, the photoreceptor is used being mounted on an electrophotographic apparatus, such as a copying machine or a laser beam printer, the surface layer of the photoreceptor is inevitably scraped off partially by a contact member such as a cleaning blade or a charging roller. In a case where the surface layer of the photoreceptor is scraped, the charge retainability of the photoreceptor lowers and images of good quality can no more be provided. Accordingly, for improving the durability of the copying machine, the laser beam printer, etc. it has been demanded for a photoreceptor having a surface layer resistant to the contact member, that is, a surface layer of high printing resistance with less amount scraped by the contact member.
In order to improve the durability of the photoreceptor by increasing the printing resistance of the surface layer, it may be considered to increase the content of the binder resin in the charge transporting layer as the surface layer. However, as the content of the binder resin in the charge transporting layer increases, it results in a problem that the light responsivity lowers. In a case where the light responsivity is low, that is, the decay speed of the surface potential after exposure is slow, since it is used repetitively in a state where the residual potential increases and the surface potential of the photoreceptor is not sufficiently decayed, the surface charges at the portion to be erased by the exposure are not erased sufficiently to result in troubles such as early lowering of the image quality.
Lowering of the light responsivity is attributable to a low charge transportability of the charge transporting substance. In the function separated photoreceptor, surface charges on the photoreceptor irradiated with a light are eliminated when charges generated in the charge generating substance by light absorption are transported by the charge transporting substance to the surface of the photoreceptor. Therefore, in a care where the content of the charge transporting substance in the charge transporting layer is lowered relatively along with increase of the content of the binder resin, the charge transportability of the charge transporting layer is further lowered when the charge transportability of the charge transporting substance is low to lower the light responsivity as described above. Accordingly, in order to prevent lowering of the light responsivity and ensure a sufficient light responsivity, a high charge transportability is required for the charge transporting substance.
Further, the size has been reduced and the speed has been increased recently in electrophotographic apparatus, for example, digital copying machines and printers, and improvement for the sensitivity has been required as the characteristics of the photoreceptor for coping with the increase of the speed, and high charge transportability has been demanded more and more as the charge transporting substance. Further, in the high speed electrophotographic process, since the time from exposure to development is short, a photoreceptor of high light responsivity is demanded. As described above, since the light responsivity depends on the charge transportability of the charge transporting substance, a charge transporting substance having a higher charge transportability is demanded also with such a view point.
As the charge transporting substance capable of satisfying such a demand, an enamine compound having a charge movability higher than that of the charge transporting substance described above has been proposed (refer, for example, to Japanese Unexamined Patent Publications JP-A 2-51162 (1990), JP-A 6-43674 (1994) and JP-A 10-69107 (1998)).
Further, a photoreceptor provided with a high charge transportability by the incorporation of a polysilane and improved with the chargeability and the film strength by the incorporation of an enamine compound having a specific structure has been proposed (refer to Japanese Unexamined Patent Publication JP-A 7-134430 (1995)).
On the other hand, in the electrophotographic process, the photoreceptor is exposed to an active gas such as ozone or NOx generated during charging by corona discharge, UV-rays contained in a light used for exposure and charge elimination, or heat. In a case where the photoreceptor is exposed to the active gas, UV-rays, or heat described above, free radicals are generated in the photosensitive layer to decompose or degrade the materials constituting the photosensitive layer. Accordingly, the charge transporting substance is required to be stable against light, heat, and active gas such as ozone or NOx as described above. However, charge transporting substance capable of satisfying such requirements has not yet been obtained and in a case where the photoreceptor is used repetitively, fatigue degradation such as lowering of the charge potential, increase of the residual potential, and lowering of the sensitivity are caused, particularly, due to the decomposition or degradation of the material consisting the photosensitive layer, particularly, the charge transporting substance to result in a problem of degradation of the image quality.
As the technique for preventing decomposition and degradation and for mitigating the fatigue degradation upon repetitive use of the charge transporting substance, etc. it has been known to add an antioxidant or a light stabilizer to the photosensitive layer (for example, refer to Japanese Examined Patent Publication JP-B2 2730744). However, when the antioxidant or light stabilizer is added to the photosensitive layer, while the fatigue degradation can be mitigated, it results in a problem of lowering the sensitivity and the light responsivity. Lowering of the sensitivity and the light responsivity develops particularly remarkably under a low temperature circumstance.
In order to suppress the lowering of the sensitivity and the light responsivity upon addition of the antioxidant, light stabilizer, etc. it may be considered to use a charge transporting substance of high charge movability. Then, it has been studied on combined use of the antioxidant and the light stabilizer with a specified charge transporting substance. For example, it has been proposed a combination of a hydrazone compound and an antioxidant (refer to Japanese Unexamined Patent Publication JP-A 64-44946 (1989)), a combination of an alkenylamine compound and an antioxidant (refer to Japanese Unexamined Patent Publication JP-A 11-271995 (1999)), and a combination of a diamine compound and a light stabilizer (refer to Japanese Unexamined Patent Publication JP-A 2001-51434).
Also in a case of using the antioxidant and the light stabilizer in combination with the specified charge transporting substance as described in JP-A 64-44946, 11-271995, and 2001-51434, its involves a problem that degradation due to repetitive use can not be improved sufficiently for those having good initial sensitivity, while the initial sensitivity and the chargeability are not sufficient for those showing a less degradation by the repetitive use.
Further, no sufficient sensitivity and light responsivity can be obtained under a low temperature circumstance even by the combined use of the antioxidant and the light stabilizer with the enamine compound of high charge transportability as described in JP-A 2-51162, 6-43674 or 10-69107. Further, in the photoreceptor described in JP-A 7-134430, while a high charge transportability is provided by the incorporation of polysilane, the photoreceptor using polysilane involves a problem that it is sensitive to light exposure and characteristic as the photoreceptor is lowered by exposure to light, for example, during maintenance even when the light stabilizer is added to the photosensitive layer.